AMTEC Cell Research Articles

Performance Evaluation on Mass Transport in Cathode-side Molybdenum Electrodes for a Low-temperature-type AMTEC Cell.

アルカリ金属熱電変換(AMTEC)のカソード側電極内の物質移動とその経時変化特性は出力性能に大きく影響する要因である. Mo電極の性能は1100 K以上の温度域で急激に劣化することが既に報告されている. しかしより低い温度域では, Mo電極の高い初期性能はより長時間維持される可能性がある. 本研究では温度範囲900-1050 Kにおいて, Na低圧蒸気雰囲気を利用する電極評価セル(SETC)による実験を実施した. 電極の限界電流密度を測定し, Na物質移動特性を自由分子流と仮定した無次元形状係数Gにより評価した. Mo電極部温度904 K, 1007 K, 1056 Kの作動条件において, 約100-150時間でGはそれぞれ20, 78, 122の一定値に収束する結果を得た. 904 Kの測定結果よりAMTEC 出力特性を検討すると, 従来のTiN電極(G = 150)と比較してMo電極は約50%の出力増加が期待できることを明らかにした.

小型化AMTECセルの性能解析(オーガナイズドセッション2 高温エネルギー変換)

This paper discusses two new concepts for miniaturized AMTEC cells. The miniaturized AMTEC cells, which have larger packed electrode area in the same volume, are expected to have a feature to increase their volumetric output power densities. The first concept is direct deposited micron-scale current leads. The Mo micron-scale current leads, directly deposited on the cathode side electrode surfaces by laser CVD method, can eliminate the necessity for the conventional tied-on metal mesh current lead structure. The second is the addition of guard pipes on beta"-alumina solid electrolytes. The use of such pipes can reduce mechanical stress of the cells. A thermal and mass transfer analysis on the configuration factors has been performed. The influence of sodium vapor pressure drops of the guard pipes was evaluated and the optimum geometrical arrangement was considered. The expected power density of the direct deposited current leads was also evaluated. Because the contact resistance can be reduced significantly, the predicted output power is 25% higher than that of the conventional mesh lead cells.

E232 小型アルカリ金属熱電変換セルの電極特性と物質移動

E232 小型アルカリ金属熱電変換セルの電極特性と物質移動

Utilization of petroleum hydrocarbons by Pseudomonas fluorescens isolated from a petroleum-contaminated soil

During extended testing at the Jet Propulsion Laboratory and the Air Force Research Laboratory, it has been found that maximum power output of the PX-3A AMTEC cell decreases with time. Starting with a peak maximum power output of 2.45 W it decreases almost continuously to 1.27 W at the end of 18 000 h. In this paper, the factors contributing to this power loss have been investigated.The cell uses a β″-alumina solid electrolyte (BASE) whose material degrades with time due to changes in the chemical and thermal conditions within the cell during operation. The analysis shows that the β″-alumina degradation manifests itself as an increase in its ionic resistance which reduces power output. The β″-alumina is responsible most of the power degradation in the first 7000 h of operation. Thereafter, though β″-alumina degradation continues to cause power loss, other components and their materials in the cell also contribute to power loss. Some suggestions are made that will help reduce the rate of power degradation and extend the useful and functional time of the cell which primarily deal with reducing the chemical contamination of the BASE, use of β-alumina instead of β″-alumina and keeping the electrode current density below a certain critical value.

Design, Analyses, and Fabrication Procedure of Amtec Cell, Test Assembly, and Radioisotope Power System for Outer-Planet Missions

This paper describes the results of OSC's department of energy (DOE)-sponsored alkali metal thermal-to-electric conversion (AMTEC) generator studies. The paper was prepared in response to an invitation from the International Astronautic Federation for the presentation of a 1-h keynote lecture on our AMTEC studies at the Space Nuclear Power and Propulsion Systems session of the 48th International Astronautical Congress. The paper covers a broad range of topics, which explains its unusual length. After presenting the background of those studies and the operational principles of the AMTEC converters, it describes a novel methodology for the coupled solution of the interdependent thermal, electrical, and fluid flow differential and integral equations governing their performance, and the application of that procedure to OSC's recommended cell design. As explained in the paper, that design was the result of parametric analyses of 35 cell design variations to determine the individual effects of 27 design parameters and various operating parameters on the cell's power output, efficiency, and critical temperatures. The analysis revealed a method of substantially improving the cell's performance by enhancing the internal reflectivity of the cell wall, and identified a specific cell wall composition and coating to achieve that goal. The paper presents a set of OSC-recommended cell design parameters for the ultimate long-service flight generators, as well as a set of parameters for an interim cell design for planned short-term ground tests. The primary difference between them is that the ultimate design employs refractory metal components to enhance compatibility with high-temperature sodium vapor, while the interim design is based on stainless steel, nickel, and Haynes-25, which are expected to be adequate for initial short-term tests, but not for long-duration missions at high operating temperatures. The paper also presents a detailed fabrication sequence for the recommended interim cell design, which is being implemented by AMPS, Inc., for a DOE-sponsored technology development program. It then describes the extension of that analytical procedure to a variety of OSC-designed radioisotope-heated generators employing the recommended cell design, with particular attention to the thermal insulation between the outside of the 16 cells and the inside of the generator housing. The studies found that the performance of the generator is optimized by employing a hybrid insulation system, in which the space between the cells is filled with fibrous Min-K insulation, and the generator walls are lined with tapered (i.e., graded-length) multifoil insulation. The paper then examines the performance of the OSC generator designs for various fuel loadings, output voltages, and mission phases, and assesses their ability to meet the stipulated temperature constraints and power demands of the Europa Orbiter and Pluto Express missions under consideration by NASA. Finally, it presents an OSC-recommended design and fabrication procedure of an electrically heated four-cell test assembly with hybrid insulation to simulate the prototypic 16-cell generator. That test assembly will be analyzed by OSC, built by AMPS, and tested by the Air Force Phillips Laboratory (AFPL), to check the validity of the analytical predictions. The reason for basing the recommended test assembly on the interim rather than the ultimate refractory metal cell design is to avoid the delays required to develop the new technology for the latter.

Alkali metal thermal-to-electric converter development

Initial impedance measurements of multi-BASE tube AMTEC cells have been accomplished. These measurements were based on theoretical analyses performed by the Jet Propulsion Laboratory. These factors are derived from a plot of the apparent charge transfer resistance, R act, as a function of the electrical potential across the beta alumina solid electrolyte. These include the morphology factor, G, the exchange current J 0 0, and α, the transfer coefficient. The factor G was given a value of 50 as a result of the inability of present equipment to achieve 1.6 V across a single electrolyte element. Average values for the temperature independent charge exchange factor, B, are also reported. These results were taken from cells with as few as 3300 h to as many as 12,000 h of operation. The results show that if the value for G is somewhat close to actual values, then J 0 0 and α are much lower than those reported in previous literature, resulting in an increase in the charge transfer resistance.

AMTEC electrochemical heat-to electricity conversion defies second law

Sacred among the mechanical engineers is the "second law of thermodynamics," which defines the maximum possible efficiency of an engine that converts thermal energy into mechanical power. The second law value is the difference between the engine's heat-source temperature and its heat-sink temperature, divided by the absolute value of the engine's heat-source temperature. For example, an engine setting on 0/spl deg/ C ice and running on steam from 100/spl deg/C boiling water is not allowed to have more than 26.8% efficiency. Power-generating violators of the second law efficiency-limit range from horses to fuel cells. They do not burn fuel to generate mechanical or electrical power. The latest second law violator is the alkali-metal thermal-to-electric converter. Its efficiency approaches 25 percent. AMTEC cells and their variations were the topics of 12 papers at the 35/sup th/ Intersociety Energy Conversion Engineering Conference (IECEC) in July 2000.

Time-dependent BASE performance and power degradation in AMTEC

During extended testing at the Jet Propulsion Laboratory and the Air Force Research Laboratory, it has been found that maximum power output of the PX-3A AMTEC cell decreases with time. Starting with a peak maximum power output of 2.45 W it decreases almost continuously to 1.27 W at the end of 18 000 h. In this paper, the factors contributing to this power loss have been investigated. The cell uses a β″-alumina solid electrolyte (BASE) whose material degrades with time due to changes in the chemical and thermal conditions within the cell during operation. The analysis shows that the β″-alumina degradation manifests itself as an increase in its ionic resistance which reduces power output. The β″-alumina is responsible most of the power degradation in the first 7000 h of operation. Thereafter, though β″-alumina degradation continues to cause power loss, other components and their materials in the cell also contribute to power loss. Some suggestions are made that will help reduce the rate of power degradation and extend the useful and functional time of the cell which primarily deal with reducing the chemical contamination of the BASE, use of β-alumina instead of β″-alumina and keeping the electrode current density below a certain critical value.

Design optimization and integration of nickel/Haynes-25 AMTEC cells into radioisotope power systems

Design optimization and integration of nickel/Haynes-25 AMTEC cells into radioisotope power systems

Performance parameters of material studies for AMTEC cell

In the recent past, a number of programs have focused on developing AMTEC technology associated with its fabrication and design. The performance level, however, achieved to-date is below the theoretical potential of this device. For improving performance characteristics we examined the sources of power loss and proposed some solution which are demonstrated with computer calculations. Two kind of losses that reduce efficiency are thermal (as a result of thermal conduction and radiation of materials) and electrical (related to the ohmic resistance of material). Each of the losses can, in principle, be reduced separately by varying current in the load. To reduce the thermal losses, the current must be increased; to reduce the electrical losses, the current in the load must be decreased. In such inversely competitive situation, an optimum value must be sought which has been achieved by applying the optimization theory. Heat losses due to radiation are reduced by increasing the current density and by reducing the emissivity of electrodes and other surfaces. Changing of some of the materials used in the cell has proved to be helpful in improving the cell performance. As a result of this overall effort, we have been able to demonstrate the improvement in the efficiency of AMTEC cell by 77%.

98/04310 Quantifying and minimizing entropy generation in AMTEC cells

98/04310 Quantifying and minimizing entropy generation in AMTEC cells

Electronic conductivity of Na β″-alumina ceramics at high temperatures

The electronic conductivity of Na β″-alumina ceramics is important for the application of this material in different energy storage and conversion devices. It is shown that at AMTEC open circuit conditions an ionic sodium transport through the Na β″-alumina membrane and the formation of metallic sodium in the condenser compartment takes place. This phenomenon is related to a residual electronic conductivity of the solid electrolyte under these experimental conditions. Electronic conductivity measurements on Mg-stabilized Na β″-alumina ceramics at relatively high temperatures (873 K ⩽ T ⩽ 1073 K) were performed using the Hebb-Wagner measurement technique in an AMTEC device under inverse polarization conditions. Electronic conductivity of about σ e=3.4 × 10 −4 ω −1 cm −1 was found at T = 1073 K. The determined activation energy of the transport of electrons is E a=1.14 eV and corresponds well to the value obtained previously for Li-stabilized Na β″-alumina ceramics. Experiments in systems with two ionically blocking molybdenum electrodes yielded lower σ e ( T)-values. This difference is associated with an increased electronic conductivity due to a chemical interaction between the Mg-stabilized Na β″-alumina ceramics and molten sodium or sodium vapour at elevated temperatures in the AMTEC cell.